Distributed acoustic sensing (DAS) offers an alternative form of fibre optic sensing to point sensors, whereby a single length of longitudinal fibre is optically interrogated, usually by one or more input pulses, to provide substantially continuous sensing of acoustic/vibrational activity along its length. The single length of fibre is typically single mode fibre, and is preferably free of any mirrors, reflectors, gratings, or change of optical properties along its length.
In distributed acoustic sensing, Rayleigh backscattering is normally used. Due to random inhomogeneities in standard optic fibres, a small amount of light from a pulse injected into a fibre is reflected back from every location along the length of the fibre, resulting in a continuous return signal in response to a single input pulse. By analysing the radiation backscattered within the fibre, the fibre can effectively be divided into a plurality of discrete sensing portions arranged longitudinally along the fibre which may be (but do not have to be) contiguous.
If a disturbance occurs along the fibre it changes the backscattered light at that point. This change can be detected at a receiver and from it the source disturbance signal can be estimated. Low noise levels and high discrimination can be obtained using a coherent optical time domain reflectometer (C-OTDR) approach as described above.
An alternative approach to DAS is based on heterodyne interferometry. In this approach light which has passed through a given section of fibre is interfered with light that has not. Any disturbance to this section of fibre causes a phase change between the two portions of light that interfere and this phase change can be measured to give a more accurate estimate of the disturbing signal than is possible with C-OTDR. The dynamic range for such a system is limited especially when sensing very long fibres and it is often desirable to use some method to increase dynamic range.
A variety of different techniques have been proposed to meet this aim. One particularly suitable example is the derivative sensing technique (DST) as set out in Applicant's co-pending WO2008/110780 to which reference is directed. This document describes a known sensor package of the type having four fibre optic sensor coils arranged between five fibre coupled mirrors. Interrogation of the sensor package is by the introduction of a pair of optical pulses, and the coils and pulses are arranged such that a series of pulses is returned, information from each sensor coil being derivable from the phase imposed on respective pulses. WO2008/110780 notes that if the change, or derivative of the phase is measured instead then this has a much smaller amplitude than the signal itself if the difference between the two times at which the signal is measured is much less than the period of the signal being measured. A system and method are then proposed which manipulates the timing of the pulses returned from the package such that they alternately contain direct or ‘normal’ phase information and derivative phase. FIG. 6 of WO2008/110780 is reproduced in the accompanying FIG. 4, and shows the combination of returned pulse trains 604 and 606 containing derivative information (at time 614 for example), interleaved temporally with the combination of returned pulse trains 602 and 608 which contain direct phase information (at time 612 for example).